Long Term Evolution (LTE) is a radio access technology (RAT) being standardized by the 3rd Generation Partnership Project (3GPP). In LTE, all services are supported through a packet switched (PS) domain. Downlink and uplink transmission in LTE use multiple access technologies—orthogonal frequency division multiple access (OFDMA) for the downlink, and single-carrier frequency division multiple access (SC-FDMA) for the uplink.
In both OFDMA and SC-FDMA, a large number of closely spaced orthogonal subcarriers are transmitted in parallel for signaling. Thus, the signaling is defined by both frequency and time components. In LTE, radio communication between a user equipment (UE) and an eNodeB is performed through defined radio frames. Each frame is 10 ms long and is divided into 10 subframes, each 1 ms long. Each subframe is subdivided into two slots each with 0.5 ms duration. Thus, a transmitted signal in each slot or subframe is defined by a resource grid of a number of subcarriers in the frequency domain and a number of symbols in the time domain.
In LTE, the radio resources for each UE is allocated in one or more physical resource blocks (PRB). Depending on the configuration, each PRB spans 12 or 24 subcarriers (also referred to as “tones”) in the frequency domain and spans one slot (0.5 ms) in the time domain (6 or 7 OFDM or SC-FDMA symbols). One OFDM (or SC-FDMA) symbol on one subcarrier is referred to as a resource element (RE).
FIG. 1 which graphically represents an example uplink/downlink radio signaling. In this example, there are 12 consecutive subcarriers per PRB in the frequency domain. With a normal spacing of 15 kHz between adjacent subcarriers, the frequency bandwidth of the PRB is 180 kHz. The PRB also spans 7 symbols (OFDM or SC-FDMA) in the time domain. The PRB is the smallest unit of radio resource assigned by the eNodeB for any UE.
No dedicated data channels are used in LTE. Instead shared transport channel resources are used in both the downlink and the uplink communications between the eNodeB and the UEs. These shared transport channel resources—the downlink shared channel (DL-SCH) and the uplink shared channel (UL-SCH)—are each controlled by a scheduler on the eNodeB that assigns different parts of the downlink and uplink shared channels to different UEs for reception and transmission, respectively.
These shared transport channels DL-SCH and UL-SCH are respectively mapped to the physical downlink shared channel (PDSCH) on the downlink OFDM subframe and physical uplink shared channel (PUSCH) on the uplink SC-FDMA subframe. Both the PDSCH and the PUSCH are used primarily for data transport, and therefore, are designed to achieve high data rates.
The OFDM and the SC-FDMA subframes also respectively include the physical downlink control channel (PDCCH) and physical uplink control channel (PUCCH). The PDCCH is used to convey UE-specific downlink control information (DCI) from the eNodeB to the UEs. Similarly, the PUCCH is used to carry uplink control information (UCI) from the UEs to the eNodeB such as channel quality indication (CQI) reports, ACK/NACK responses, and scheduling requests (SR).
In addition to the physical channels PDSCH and PDCCH, the OFDM subframe also carries other physical channels including the physical broadcast channel (PBCH), the physical multicast channel (PMCH), the physical control format indicator channel (PCFICH), and the physical hybrid ARQ indicator channel (PHICH). The physical channels carry data from higher layers.
The OFDM subframe further carries physical signals, which are generated in layer 1 for use in system synchronization, cell identification, and radio channel estimation. The physical signals include the primary synchronization signal (P-SCH), the secondary synchronization signal (S-SCH), and the reference signal (RS). Unlike the physical channels, the physical signals are not used to carry data originating from higher layers.
In general, the physical channels PDCCH and the PDSCH occupy a significant majority of the OFDM subframe. Thus, for simplicity, the OFDM subframe in FIG. 2 is illustrated as being shared by the PDCCH and the PDSCH.
The PDCCHs are transmitted in a control region of every OFDM subframe. The control region comprises a first few (1, 2, 3 or 4) OFDM symbols of the downlink subframe, and is divided into one or more control channel elements (CCE). The number of CCEs available for PDCCHs depends on configuration parameters such as bandwidth and the number of OFDM symbols for the control region. Each PDCCH is sent on an aggregation of 1, 2, 4 or 8 CCEs. The downlink subframe not used for PDCCH can be used for PDSCH to carry data.
On the uplink, physical channels and signals are carried on the SC-FDMA frame. The roles of the uplink physical channels and signals are similar to the downlink counterparts in that the physical channels are used to carry data originating from the higher layers and the physical signals are used for layer 1 purposes. In addition to the PUSCH and the PUCCH, the uplink physical channels include the physical random access channel (PRACH). The uplink physical signal includes the reference signal (RS).
FIG. 3 illustrates a simplified view of the SC-FDMA subframe. The view is simplified in that only the PUSCH and PUCCH resources are illustrated. That is, the SC-FDMA subframe is viewed as being shared primarily between the PUCCH and PUSCH. The PUSCH resource occupies the middle subcarriers of the SC-FDMA frequency spectrum. On the spectrum band edges, a control region is located on which the PUCCH is transmitted. The PUCCH carries uplink control information including CQI reports, SR, and ACK/NACK (also referred to as HARQ-ACK).
To enable efficient resource utilization on the PUCCH, the SR, CQI, and ACK/NACK responses of several UEs are multiplexed on the PUCCH through code division multiplexing (CDM). This allows several UEs to share one PUCCH PRB. As an example, for an SC-FDMA subframe with normal cyclic prefix (CP), up to 12 different UEs may share one PUCCH PRB for CQI reporting. For the extended CP, 8 UEs may share one PRB also for CQI reporting. For SR and ACK/NACK responses, up to 36 different UEs may share one PRB for the normal CP, and 24 for the extended CP.
There are problems related to the use of CDM on the PUCCH. The planned deployment for the LTE is a reuse-one network in which the same frequency band is reused for each cell in the network. In a reuse-one network, inter-cell interference is typically a limiting factor. Moreover, time dispersion may lead to intra-cell interference. Both interference types grow with the number of utilized resources, i.e., CDM codes, per PRB. If not controlled, the interference may lead to seriously degraded overall system performance.
The 3GPP standardized solution provides only one option to reduce the load per PUCCH PRB for SRs and ACK/NACK feedback reports. The option is to make only one half or one third of the resources available for use. In other words, PUCCH is allocated, but not all of the allocated resource is used for PUCCH transmission.
One disadvantage of the existing standardized load control is that the throughput on the PUSCH is reduced. Since the SC-FDMA subframe is essentially a fixed resource, more of the subframe resource dedicated to PUCCH means that there is less resource available for PUSCH. Configuring the PUCCH with this option effectively increases the bandwidth reserved for PUCCH by a factor of two or three, which can significantly reduce the bandwidth for the PUSCH. In addition, the standardized load control is unlikely to reduce the interference level sufficiently.